A spin-on filter is designed for a specified service life. The filter is then discarded and replaced with a new filter. Typically, more than one manufacturer produces filters which are interchangeable. As a disposable or throw-away type of item, the decision to purchase one brand of filter over a different brand is often based substantially on the price of the filter, and how often it must be replaced. For filters providing lubrication oil to an engine which will need to be started at cold ambient temperatures, it is also important to select a filter that has a low flow resistance during cold start conditions, so that an adequate flow of filtered lubricant can be supplied while the engine is coming up to operating temperature.
In order to provide a high overall filtering efficiency of the filter, it is a common practice to incorporate two separate filtering elements within a common housing of the spin-on filter. Typically, one of these filters, known as the full flow filter, is used for filtering all or most of the fluid passing through the housing of the filter. The other filter element, known in the industry as a bypass filter, is used for performing additional filtration of a small portion, typically about 10 percent, of the fluid passing through the housing.
In prior filters of this type, the full flow filter generally includes a full flow filter media having a rated efficiency for removing particles of a given size from the fluid passing through the full flow filter, and the bypass filter includes a bypass filter media having a higher rated efficiency for removing particles of the given size. Stated another way, the bypass filter media is essentially denser than the full flow filter media, making it capable of removing a higher percentage of particles of the given size from the smaller portion of fluid passing through the bypass filter. Theoretically, as the fluid is continuously circulated through the spin-on filter, in sequential passes through the filter housing, all of the fluid will eventually pass through the higher efficiency media of the bypass filter to receive additional filtration.
Typically, prior filters of this type have included a venturi tube that is used to locally reduce the pressure in the fluid, at a strategic point within the housing, to aid in pulling a small portion (about 10%) of the fluid through the relatively dense bypass filter. The reduced pressure is created by directing most of the fluid flowing through the housing through a throat in the venturi tube, to thereby accelerate the fluid at the throat of the venturi tube. This acceleration of the fluid causes the fluid pressure at the throat of the venturi tube to drop, due to well known principles of fluid dynamics.
In one prior approach to providing such a spin-on filter, the full flow and bypass filters are arranged in a parallel flow relationship to one another, with the flow through the full flow filter being accelerated in the venturi tube for dropping the pressure, in such a manner that a small portion of the total flow through the filter housing is induced to flow through the relatively dense bypass filter, rather than taking the path of lower resistance through the full flow filter. The small portion of fluid passing through the bypass filter is then reunited with the larger portion of fluid that was directed through the throat of the venturi tube, and the combined flow is returned to the system through the outlet of the spin-on filter. With this arrangement, the portion of the fluid receiving higher efficiency filtration in the bypass filter bypasses the full flow filter on that particular pass through the spin-on filter. This general approach is disclosed in U.S. Pat. No. 5,906,736 to Bounnakhom, et al.
In another prior approach, the bypass filter is disposed within the filter housing in such a manner that all fluid passing through the housing of the spin-on filter, on a given pass, is first filtered by full flow filter. The filtered fluid then splits, with a larger portion being routed through the throat of a venturi tube, which is arranged for drawing a small portion of the fluid filtered by the full flow filter, on that particular pass, through a higher efficiency bypass filter element, to thereby provide a second stage of filtering for the small portion of fluid passing through both the full flow and bypass filters. This approach is disclosed in U.S. Pat. No. 6,478,958, to Beard, et al., which is assigned to the assignees of the present invention.
Although prior filters, using venturi tubes to draw a small percentage of the total fluid being filtered through a higher efficiency bypass filter, generally work well for their intended purpose, certain drawbacks have been noted. Only a small percentage of the total fluid flow is routed through the higher efficiency bypass filter on each pass of the fluid through the filter. It can thus take some time before all of the fluid passes through the bypass filter. Such filters often exhibit fairly high resistance to fluid flow through the filter during cold start operation. It has been noted that the media in such filters tends to capture contaminant particles in a non-uniform manner, which tends to reduce the amount of contaminant that the filter can remove and hold, before the pressure drop through the filter apparatus rises to a point that the filter should be replaced. The complexity of such filters also tends to undesirably drive up the cost of production and the price of the filter apparatus.
It is desirable, therefore to provide an improved apparatus and method for fluid filtration that overcomes one or more of the drawbacks cited above, or other drawbacks, problems or deficiencies in prior art filters of the type addressed by the invention.